Science paper

Chapter 8

The Cellular Basis of Reproduction and Inheritance

The large, complex chromosomes of eukaryotes duplicate with each cell division
 Eukaryotic chromosomes are composed of chromatin
– Chromatin = DNA + proteins – To prepare for division, the chromatin becomes highly compact, and the chromosomes are visible with a microscope – Early in the division process, chromosomes duplicate– Each chromosome appears as two sister chromatids, containing identical DNA molecules – Sister chromatids are joined at the centromere, a narrow region
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Sister chromatids

Centromere

Figure 8.3B_1

Chromosomes

DNA molecules

Chromosome duplication Sister chromatids

Centromere

Chromosome distribution to the daughter cells

DNApacking in eukaryotic chromosomes
Metaphase chromosome Tight helical fiber (30-nm diameter) Linker “Beads on a string” Histones Nucleosome (10-nm diameter)

DNA double helix (2-nm diameter)

Supercoil (300-nm diameter)

700 nm

Animation: DNA Packing

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Chromosomes are matched in homologous pairs
 Somatic cells have pairs of homologous chromosomes, receiving one member of each pairfrom each parent  Homologous chromosomes are matched in
– Length – Centromere position – Gene locations
– A locus (plural, loci) is the position of a gene – Different versions of a gene may be found at the same locus on maternal and paternal chromosomes
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Chromosomes are matched in homologous pairs
 The human sex chromosomes X and Y differ in size andgenetic composition  Pairs of autosomes have the same size and genetic composition  Applying Your Knowledge
– Humans have 46 chromosomes; how many homologous pairs does that represent? – If there is one pair of sex chromosomes, how many pairs of autosomes are found in humans?
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Prokaryotes reproduce by binary fission
 Binary fission means―dividing in half‖
– Occurs in prokaryotic cells – Two identical cells arise from one cell – Steps in the process
– A single circular chromosome duplicates, and the copies begin to separate from each other – The cell elongates, and the chromosomal copies separate further – The plasma membrane grows inward at the midpoint to divide the cells

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Figure 8.2A_s1Plasma membrane Cell wall

Prokaryotic chromosome Duplication of the chromosome and separation of the copies

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Figure 8.2A_s2

Plasma membrane Cell wall

Prokaryotic chromosome Duplication of the chromosome and separation of the copies

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Continued elongation of the cell and movement of the copies

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Figure 8.2A_s3

Plasma membrane Cell wall

Prokaryoticchromosome Duplication of the chromosome and separation of the copies

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2

Continued elongation of the cell and movement of the copies

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Division into two daughter cells

The cell cycle multiplies cells
 The cell cycle is an ordered sequence of events for cell division  It consists of two stages
– Interphase: duplication of cell contents
– G1—growth, increase in cytoplasm –S—duplication of chromosomes – G2—growth, preparation for division

– Mitotic phase: division
– Mitosis—division of the nucleus – Cytokinesis—division of cytoplasm
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Figure 8.4

G1 (first gap)

S (DNA synthesis)

M G2 (second gap)

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Cell division is a continuum of dynamic changes
 Mitosis progresses through a series of stages
– Prophase –Prometaphase – Metaphase – Anaphase – Telophase

 Cytokinesis often overlaps telophase

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Cell division is a continuum of dynamic changes
 A mitotic spindle is required to divide the chromosomes
– The mitotic spindle is composed of microtubules – It is produced by centrosomes, structures in the cytoplasm that
– Organize microtubule arrangement –...